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  • #16
    I wonder what would be the changes needed to the tail for it to be better I rather liked the design and would like to use a tail similar to the Arrowcopter and ELA G10.
    Is the design flawed for gyro use?

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    • #17
      Originally posted by JasonS View Post
      I wonder what would be the changes needed to the tail for it to be better I rather liked the design and would like to use a tail similar to the Arrowcopter and ELA G10.
      Is the design flawed for gyro use?
      The static longitudinal stability for BCAR Sec T is very similar to ASTM standard for airplanes or trikes except it gives an allowance of returning to within 15% of datum airspeed instead of 10% which is even easier than trikes and airplanes. It also asks that the control force versus speed curve must remain positive from Vmin to Vdf and its has a requirement to test a variable pitch trim system at different datum trim airspeeds and test from that with trim engaged.

      For dynamic stability these excursions from datum airspeed (ranging from Vmin to Vdf) when they are causing oscillations should be damped with controls fixed and free both for longitudinal and lateral stability.

      That's really it. Obviously something did not work in the above or the similar testing in lateral and/or directional stability were questionable. But "Phugoids" point to longitudinal stability really. These criteria are the same for stability testing in other categories of aircraft as well. No magic there.

      What changes would be needed to the tail could range from larger span or larger vertical winglets t slight incidence change or to move the tail slightly further back. Any of these changes are available to experiment in design and testing. This does not mean that someone else cannot use this tail configuration on their design. You cannot think of the tail in isolation. Its interaction and effect of the whole aircraft system is what determines its effectiveness and its not working in a vacuum. I guess some can say that V tails are much more difficult to make effective in a gyroplane. I have heard something like that and it may be true. I have never liked V tails anyway and never thought about them.
      Last edited by fara; 06-08-2018, 04:00 PM.

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      • #18
        Is the tail on say a Dominator that much different then a T tail design?
        I realize the Dominator tails are controlled as a whole since the whole thing moves as opposed to just a rudder only so how does the T tail design differ in stability vs. a T tail?
        I never considered a V tail as an option either by the way.
        Thankx for your thoughts.

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        • #19
          Originally posted by JasonS View Post
          Is the tail on say a Dominator that much different then a T tail design?
          I realize the Dominator tails are controlled as a whole since the whole thing moves as opposed to just a rudder only so how does the T tail design differ in stability vs. a T tail?
          I never considered a V tail as an option either by the way.
          Thankx for your thoughts.
          It doesn't take long to do a simple set of tests on any design you want for stability stick free or fixed. You have t do it across the speed range from the slowest to past the Vne. I suggest you do those tests on a Dominator in 10 knot increments and in half an hour you'll know if Dominator tail (cruciform shape close to the prop) is better. The case to check is with minimal weight in front seat. That usually will produce most adverse conditions for this. Do this test power on and power off. I have my own opinions on it and you can come up with yours. Stability however is not an opinion. Its a boring simple test and the results are results. From what I know in terms of damping a tail farther back is better. I have not seen a cruciform tail farther back so the comparison cannot be fair apples to apples. In fact cruciform tail really is restricted in that it can't go that far back where as other tails can making them more effective in HS sense. Their HS also tends to be wider than cruciform tails. So its obvious what would dampen more but again its really not an apples to apples comparison because of limitations of placement for one tail over the other. Its also not necessary to go overboard once sufficient stability is achieved.
          Generally in airplane design you do not want tails in slipstreams or reduce that. This is to reduce structural buffeting fatigue and to reduce in cabin noise due to buffeting but in a gyro, noise is not the concern because cabins are usually detached unless design dictates otherwise. The other disadvantage of tails in slipstream is trim changes needed with different settings of engine power. These are all considerations you need to think and compromise on as a designer.

          In general the lever arm of the empennage should be as long as possible so one can reduce its size and drag but we are not talking of high performance airplanes here so I personally don't care about this in a gyro and rather go towards extra volume in the tail. In a T-tail one advantage related to this is that as a rule of thumb, you can reduce tail volume by about 5% for both vertical and horizontal planes compared to conventional airplane tail (like Sport Copter) because of end plate effect and get away with smaller tail (thus smaller drag) in theory. I have also seen T-tails in gyroplanes incorporate what one may consider a sort of a dorsal fin and that helps in yaw as well, although it was clear whoever designed it was not really thinking about that, it just came about due to structural considerations.

          Sweep angle of vertical tail for low speed aircraft should be kept below 20 degrees. A large sweep angle will reduce max. lift coefficient of it. Symmetrical airfoils must be chosen for vertical stab like NACA 0009, 0012 etc. Incidence angles are usually 2 to 3 degrees negative for HS and a dihedral angle can be chosen for the tail (HS) for certain effects if designer so desires. There are many things to mess around with if you want to go design something new or take an existing design and improve it by making changes in airfoils, lever arm, aspect ratio and incidence of the tail. But it can also get ugly if you do the wrong thing. Rudder usually should be allowed to go 25 to 35 degrees and acts as a flap to the vertical stab and the max lift can be calculated on it just like a plain flap. Usually the mean chord of the rudder is 40-50% of the effective chord of the vert tail but in a gyroplane we use much more than that. In AR-1, this ratio is almost swapped backwards on its head, which increases its hinge moment at 90+ knots for sure and may be I'll work on reducing it in next iteration with horn incorporation. In case of cruciform tail with full moving surface (whole thing is deflected to create a yawing force) the required tail volume can be dropped generally by 15% relative to vert stab and rudder.
          Good luck
          Last edited by fara; 06-09-2018, 08:56 PM.

          Comment


          • #20
            Originally posted by fara View Post

            It doesn't take long to do a simple set of tests on any design you want for stability stick free or fixed. You have t do it across the speed range from the slowest to past the Vne. I suggest you do those tests on a Dominator in 10 knot increments and in half an hour you'll know if Dominator tail (cruciform shape close to the prop) is better. The case to check is with minimal weight in front seat. That usually will produce most adverse conditions for this. Do this test power on and power off. I have my own opinions on it and you can come up with yours. Stability however is not an opinion. Its a boring simple test and the results are results. From what I know in terms of damping a tail farther back is better. I have not seen a cruciform tail farther back so the comparison cannot be fair apples to apples. In fact cruciform tail really is restricted in that it can't go that far back where as other tails can making them more effective in HS sense. Their HS also tends to be wider than cruciform tails. So its obvious what would dampen more but again its really not an apples to apples comparison because of limitations of placement for one tail over the other. Its also not necessary to go overboard once sufficient stability is achieved.
            Generally in airplane design you do not want tails in slipstreams or reduce that. This is to reduce structural buffeting fatigue and to reduce in cabin noise due to buffeting but in a gyro, noise is not the concern because cabins are usually detached unless design dictates otherwise. The other disadvantage of tails in slipstream is trim changes needed with different settings of engine power. These are all considerations you need to think and compromise on as a designer.

            In general the lever arm of the empennage should be as long as possible so one can reduce its size and drag but we are not talking of high performance airplanes here so I personally don't care about this in a gyro and rather go towards extra volume in the tail. In a T-tail one advantage related to this is that as a rule of thumb, you can reduce tail volume by about 5% for both vertical and horizontal planes compared to conventional airplane tail (like Sport Copter) because of end plate effect and get away with smaller tail (thus smaller drag) in theory. I have also seen T-tails in gyroplanes incorporate what one may consider a sort of a dorsal fin and that helps in yaw as well, although it was clear whoever designed it was not really thinking about that, it just came about due to structural considerations.

            Sweep angle of vertical tail for low speed aircraft should be kept below 20 degrees. A large sweep angle will reduce max. lift coefficient of it. Symmetrical airfoils must be chosen for vertical stab like NACA 0009, 0012 etc. Incidence angles are usually 2 to 3 degrees negative for HS and a dihedral angle can be chosen for the tail (HS) for certain effects if designer so desires. There are many things to mess around with if you want to go design something new or take an existing design and improve it by making changes in airfoils, lever arm, aspect ratio and incidence of the tail. But it can also get ugly if you do the wrong thing. Rudder usually should be allowed to go 25 to 35 degrees and acts as a flap to the vertical stab and the max lift can be calculated on it just like a plain flap. Usually the mean chord of the rudder is 40-50% of the effective chord of the vert tail but in a gyroplane we use much more than that. In AR-1, this ratio is almost swapped backwards on its head, which increases its hinge moment at 90+ knots for sure and may be I'll work on reducing it in next iteration with horn incorporation. In case of cruciform tail with full moving surface (whole thing is deflected to create a yawing force) the required tail volume can be dropped generally by 15% relative to vert stab and rudder.
            Good luck
            Excellent post my friend. I'll save that and post it on PRA's web-site for others.
            Resistance is futile…… You will be compiled!
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            John Rountree

            PRA- Director, Secretary
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            PRA31 - Vice President of S.D. Rotorcraft Club
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